In the technical field of color modulation, it is a key technique to dispense quantitative colorant by means of a reciprocating pump. At present, piston pumps, plunger pumps, folding pumps, etc., are common reciprocating positive displacement pumps. A strict requirement is made to the volume of colorant in color modulation, i.e. a requirement that an error of a dispensed volume of colorant cannot exceed a certain percentage (e.g., less than 1%) when a pump dispenses a large volume of colorant (e.g., greater than 15 ml), and an error of the dispensed volume of colorant cannot exceed a certain percentage (e.g., less than 12%) either with regard to the dispensing of a small volume of colorant (e.g., less than 0.5 ml). However, currently common reciprocating pumps for dispensing a large volume of colorant are high in dispensed volume per stroke and therefore, it may lead to great dispensing errors during dispensing of a small volume of colorant in spite of small errors in the stroke of the pumps, thus resulting in abnormal colors that cannot meet the requirements of products. Consequently, on the control standard for the colorant dispensing accuracy of the colorant dispensers, there is an undesirable situation that the accuracy requirement (e.g., 12%) for dispensing a small volume of colorant is lower than that (e.g., 1%) for dispensing a large volume of colorant. To solve this problem, in particular to reduce the errors in dispensing of a small volume of colorant, a stepper motor is usually used to realize the stroke of a reciprocating pump. However, the number of micro-steps of the stepper motor is limited. At present, a principal solution is disposing two colorant dispensers at the same time, one being a colorant dispenser essentially consisting of a large pump and mainly used for dispensing a large volume of colorant and the other one being a colorant dispenser essentially consisting of a small pump for dispensing a small volume of colorant. In this way, the volume of colorant dispensed by the control pump may be well controlled, but the disposing of two colorant dispensers not only costs much more, but also needs more space with inconvenience in use; therefore, this does not meet the requirements of intensive production in modern economy, and especially is not suitable for use in stores. Another solution is to dispose a pair of pumping-out systems for both large volume dispensing and small volume dispensing in parallel by connecting a large pump having a large-section cylinder body with a small pump having a small-section cylinder body in parallel in the same pump. This also involves the problems of large space occupation, complicated structure and high cost. Patent No. US2009236367A1 discloses a dual pump with two paint outlets, wherein springs are disposed in the pump to achieve the objective of disposing both a large volume and a small volume of colorant using one system; the springs are compressible such that the objective of dispensing a large amount of colorant is achieved when the pump presses down; further compression is carried out after a pillar abuts the bottom, and a thin rod continues to move downwardly to enter the piston chamber, thereby achieving the objective of high accuracy during dispensing of a smaller amount of colorant. The above prior art described above has the disadvantage of the above-mentioned structure being complicated with high cost, and the solves the problems by increasing a small dispensed colorant volume per stroke under the premise of guaranteeing high efficiency of dispensing of a large volume of colorant, such that a basically quantitative absolute error is reduced relative to the small dispensed colorant volume per stroke by the reciprocating lever so as to improve the accuracy of dispensing of a small volume of colorant. For example, a theoretical dispensed colorant volume per step of a stepper motor is reduced to improve the accuracy. In addition, it is general to improve the accuracy of disposing a small volume of colorant in this way in this industry.
In addition to the above-mentioned disadvantages of the prior art and its emphasis in problem solving as mentioned above, the product of patent No. US2009236367A1 also has the following defects: (1) when the pumps are not used for a long time, and thick colorant cannot be pressed down as usual and needs an increased force to press it down, the springs may be easily deformed under compression during dispensing of a large volume of colorant due to a certain elastic limit of the springs and certain friction between the piston and the wall, resulting in errors in the dispensing stroke of the pump; (2) a requirement of remarkably good leakproofness is made to the sliding device, whether the middle or the two sides, to avoid colorant leakage; (3) the pre-tightening force of the spring must be set; when the pre-tightening force of the spring is set to be greater, excessive motor power may be consumed, leading to energy waste; meanwhile, in dispensing of a small volume of colorant after the pump abuts the bottom, too much force applied to the pump body against the action of the spring may easily cause deformation, shifting and vibration of the frame and lead to changing of the stroke of the pump rod that may impact the accuracy of the dispensed volume; (4) when the pre-tightening force of the spring is set to be smaller with great resistance of colorant, it may easily cause compression of the spring, and therefore, the accuracy of the dispensed volume may also be influenced; and (5) air in the pump may seriously affect the accuracy of dispensing of a small volume of colorant.